Project 3 focuses on the role of three hypothalamic neuropeptides in drug addiction. The peptides, melanocyte-stimulating hormone (MSH), melanin-concentrating hormone (MCH), and orexin (also known as hypocretin), have all been implicated in the control of feeding. Hypothalamic neurons expressing these peptides project to the nucleus accumbens (NAc), a brain region important for the rewarding actions of drugs of abuse. In addition, orexin-containing neurons project to the ventral tegmental area (VTA), also important for drug reward, and to the locus coeruleus (LC), which is implicated in physical opiate dependence and withdrawal. In addition, both MCH and orexin are expressed in the lateral hypothalamus (LH), a brain region known to be important in reward. We hypothesize that the molecular and neuronal pathways defined by these peptides and their receptors may play an important role in regulating an individual's responses to many types of rewards, including drugs of abuse. Preliminary data support this hypothesis, as exposure to drugs of abuse modifies the expression of the peptides or their receptors in the hypothalamus or NAc. Moreover, modulation of these pathways via pharmacological or genetic tools alters drug responses. Specifically, MSH acts via its receptor (MC4-R), enriched in the NAc, to sensitize an animal to the behavioral effects of drugs, and chronic drug exposure increases MC4-R expression in this brain region. Likewise, MCH acts via its receptor (MCH-R), also enriched in the NAc, apparently to also sensitize an animal to the behavioral effects of drugs, although drug exposure downregulates MCH-R expression in the NAc. Orexin is required for normal opiate withdrawal behaviors, and its expression is induced during withdrawal. These data provide the first glimpse into a series of novel hypothalamic circuits that appear to be highly relevant to the molecular neurobiology of drug addiction. Both genetic and pharmacological tools will be used to further elucidate the role of these peptide pathways in drug addiction. Regulation of peptide expression via transcription factors, including CREB, will also be explored. Novel transgenic mouse lines will be generated to specifically test the role of peptide signaling in the NAc and elsewhere in modulating neuronal and behavioral responses to drugs of abuse. Molecular mechanisms of action of the peptides will be investigated by better defining their target neurons in the NAc, VTA, and LC, and characterizing the signaling pathways through which the peptides produce their effects. Together, these highly integrated studies will shed new light on neural and functional connections between the hypothalamus and the brain's reward circuitry and other drug-responsive brain regions, and define the contribution of these connections to drug abuse and addiction.